Physical interactions between DinI and RecA nucleoprotein filament for theregulation of SOS mutagenesis

The Escherichia coli dinI gene is one of the LexA-regulated genes, which ar e induced upon DIVA damage. Its overexpression conferred severe UV sensitiv ity on wild-type cells and resulted in the inhibition of LexA and UmuD proc essing, reactions that are normally dependent on activated RecA in a comple x with single-stranded (ss)DNA. Here, we study the mechanism by which DinI inhibits the activities of RecA. While DinI neither binds to ssDNA nor prev ents the formation of RecA nucleoprotein filament, it binds to active RecA filament, thereby inhibiting its coprotease activity but not the ATPase act ivity. Furthermore, even under in vitro conditions where UmuD cleavage depe ndent on RecA-ssDNA-adenosine-5'-(3-thiotriphosphate) is blocked in the pre sence of DinI, LexA is cleaved normally. This result, taken together,vith e lectron microscopy observations and linear dichroism measurements, indicate s that the ternary complex remains intact in the presence of DinI, and that the affinity to the RecA filament decreases in the order LexA, DinI and Um uD. DinI is thus suited to modulating UmuD processing so as to limit SOS mu tagenesis.